1. Field of the Invention
The present invention relates to a sewing machine which gives a warning signal when the amount of under thread has reduced to a predetermined value.
2. Description of the Prior Art
In the usual sewing operation, upper or needle thread is supplied from a relatively large bobbin, while the under thread is supplied from a relatively small bobbin located beneath the sewing plate. Therefore, the operator has to frequently check the under thread bobbin to see if the amount of remaining thread is sufficient to complete a stitch without interruption.
Japanese Pat. No. 72-17586 discloses a device for detecting when the amount of remaining under thread reaches a predetermined value to give a warning signal. This device comprises a rotary hook having an opening therein and a bobbin case having an identical opening. When the openings of the rotary hook and the bobbin case are aligned to each other on a particular phase during rotary motions, a spring-loaded probe is automatically inserted into the aligned openings to make contact with the circumference of the under thread wound on the bobbin cylinder. When the amount of the under thread has reduced to a predetermined value the probe triggers a warning circuit to alert the operator. This probe is mounted on a pivot which rotates in synchronism with the rotary motion of the rotary hook so as to give a rocking movement to the probe. Since the probe is brought into contact with the under thread in the radial direction, this tends to give a considerable amount of impact to the under thread tension, so that the latter is drawn up with an increased tension when the probe is in contact therewith, which tends to give unsatisfactory results.